In a fingerprint input transducer or sensor, the finger under investigation is usually pressed against a flat surface, such as one side of a glass plate, and the ridge-valley pattern of the finger is sensed by some sensing means such as an interrogating light beam. The processing of the fingerprint information thus obtained may comprise laser techniques. Such fingerprint identification devices are generally used to control the access of individuals to information, for instance computer terminals (information access control) or buildings (physical access control).
One of the problems associated with fingerprint sensors concerns the reliable sensing of weak fingerprints. "Weak fingerprints" are fingerprints which have a small modulation depth on the sensing surface, or in other words, a small amount of topographic relief. A weak fingerprint may occur when the finger under test is not firmly pressed against the surface, when the finger does not have sufficient fluid such as oil or moisture to leave sufficient traces on the surface, etc.
Therefore, there is a need for a fingerprint input sensor or transducer which is adapted to sense not only fingerprints of regular quality, but also weak fingerprints of small modulation depth. There is also a need for a method for enhancing fingerprints.
The U.S. Pat. No. 4,053,228 discloses a finger identification system in which a fingerpress is formed by pressing a finger against the back surface of a transparent glass plate and holding it in a predetermined position thereon. This fingerpress is interrogated by a light beam directed through the front surface of the glass plate. The interrogating beam is partially reflected at the back surface of the glass surface to provide a signal beam, carrying fingerpress information. The back surface of the glass plate is coated with a coating to enhance the difference in reflectivity of the valleys and crests of the fingerpress. In particular, the coating on the back surface enhances the difference in reflection at that surface between those areas where the crests of the finger are in intimate contact with the back surface of the plate and those areas under the valleys of the finger, where air is in contact with the back surface of the plate. In this fingerprint transducer an enhanced fingerpress will only be created as long as the finger engages the back surface of the plate, that is for a short while.
U.S. Pat. No. 4,120,585 discloses a pliable, resilient prism for use in an optical imaging system such as a fingerprint reader. The base surface of the prism is contacted by the finger under investigation. The pliable prism deforms under the pressure of the contacting finger. Light from a light source passes through one side face of the prism to the base surface from where it is reflected and passed through the other side face to a photosensitive element. This element is provided for actuating the optical imaging system. If there is sufficient finger pressure, an air gap will be created between the base surface and a housing, resulting in an additional area the light beam of which is directed to the photosensitive element. This element will activate the system by closing of a switch. In this fingerprint system, no means are discussed to process weak fingerprints.
In SPIE, Vol. 185, Optical Processing Systems (1979), pp. 86-92, an elastomer storage device is disclosed which uses a photoconductor. A glass substrate is chemically etched to form a transparent conductive ground plane. The ground plane is coated with the mentioned photoconductor and overcoated with a thin elastomer layer. A charge plane is positioned several mils above, and parallel to, the elastomer surface. The resulting gap is filled with low pressure argon. In operation, a high voltage source is connected between the charge plane and the ground plane, producing an electrostatic force across the elastomer and photoconductor layers. The electrostatic force is normal to the elastomer surface and is proportional to the electric field strength at each point. Because the elastomer is incompressible and the force is uniform, no deformation occurs. When the photoconductor is exposed to a light distribution, charge carriers are photogenerated and move in the electric field to the elastomer-photoconductor interface where they are trapped. The trapped charges form an electrostatic surface charge layer in which the charge density distribution is directly proportional to the exposure distribution of the input signal. The resulting electric field distribution causes the elastomer to deform creating a surface relief pattern which is directly related to the exposure distribution through the surface charge density. The deformation will continue as long as the non-uniform charge distribution is maintained. This storage device, incidentally, is not contemplated for use as a fingerprint transducer.
In SPIE, Vol. 123, Optical Storage Materials and Methods (1977), pp. 32-36, a thermoplastic data storage medium is disclosed. This medium is a multilayered device consisting of a thermoplastic layer, a photoconductor layer, and a transparent conductive layer coated on a glass or flexible polyester substrate. The optical data storage in thermoplastic is based on the principle that thermoplastic deforms under stress when heated to an appropriate temperature. During writing, a uniform charge is applied to the thermoplastic surface. The device is then exposed to a holographic pattern which alters the conductivity of the photoconductor and thus the surface charge distribution. This non-uniform charge distribution results in electrostatic forces which deform the thermoplastic upon heating to a critical softening/developing temperature. When the sample cools, the deformation corresponding to the holographic pattern remains and the information can be retrieved by illumination with a reference beam. Also this storage medium is not contemplated for use as a fingerprint transducer.
Similar optical recording and storage devices are known from SPIE, Vol 123, Optical Storage Materials and Methods (1977), pp. 10-16, and pp. 74-77.